Image processing software provides a user with a variety of tools with which to modify digital images. Typically, such software includes filter effects (e.g. black and white, colour effects or stylisations), and adjustments (e.g. brightness, saturation, contrast adjustments) which are applicable to the whole of an image being edited. Also, selective image modifications are typically provided which can be applied by brushes or other area selection tools. More advanced effects may use an algorithmic approach to identify areas of the image for modification automatically and applying the adjustment. Such algorithmic effects include, for example, automatic red-eye or blemish removal effects.
Each effect or adjustment typically includes a number of user controllable parameters that control the output of the effect and can be edited by a user. Image editing software will typically allow the user to remove or add effects and modify their parameters. Conventionally, when an effect is modified or removed, the original image is reprocessed by applying all the effects that have been selected taking into account any modification of parameters by the user. As will be appreciated, although this allows the user complete freedom to apply, modify or remove effects, it is computationally inefficient because every effect must be reprocessed when the user makes a change.
To improve on this approach some image editing applications record in memory a history of user modifications to the image being edited. This is achieved by storing in memory a different rendered image after each modification made to an image by the user. For example, a cache can be kept containing a chronological list of images corresponding to each modification made to the image. The user can then jump or step back through the cached images to revert to an earlier version of the image. However, this has the disadvantage that the device on which the image editing software is being executed has to store a large number of cached images. This consumes a large amount of memory which can lead to degraded performance of the application.
These computational inefficiencies are particularly problematic when the software is being executed on a device with limited computational resources such as a tablet device, cell phone or other mobile electronic device.